The following code behaves differently between .NET 4.8 and .NET 6.0 with 4.8 behaving as I'd expect (see also Why is "ss" equal to the German sharp-s character 'ß'?)
Thread.CurrentThread.CurrentCulture = new CultureInfo("de-de");
Thread.CurrentThread.CurrentUICulture = new CultureInfo("de-de");
var eq = "ß".Equals("SS", StringComparison.CurrentCultureIgnoreCase);
// True in .NET 4.8
// False in .NET 6.0